1. Field of Invention
This invention relates to optical signal processing. More particularly, this invention relates to amplification of optical signals using a semi-conductor component.
2. Description of Related Art
Optical amplifiers amplify optical signals prior to, or during, transmission of the signal over an optical network.
In optical networks, many fiber links are loss limited. One cause is the limited power available from typical laser diodes. Optical fiber non-linearities may also limit the transmission power of optical signals. This limited power, combined with the losses in the fiber and other system components, restricts the length of fiber that can be used without optical amplification.
Optical amplifiers increase the power level of an optical signal beam without conversion to the electrical domain. For example, gains of 30 dB are attainable at 1550 nm using semiconductor-optical amplifiers (SOAs).
As shown in FIG. 1, there are a number of possible locations for optical amplifiers 100 in an optical network. An optical amplifier 100 just following a transmitter increases the power traveling down the fiber. Optical amplifiers 100 along the fiber path continually keep the power levels above system noise. An optical amplifier 100 located at the fiber end acts as a receiver pre-amplifier, enhancing its sensitivity. Many optical amplifiers can be placed in a fiber network to extend the total path length to thousands of kilometers.
One particular type of optical amplifier commonly used by the telecommunication industry is an Erbium Doped Fiber Amplifier (EDFA). However, EDFAs are complex, expensive to produce and amplify over a limited range of wavelengths.
Amplification in an EDFA is provided by optically pumping erbium atoms within the fiber of the EDFA. These atoms provide gain over a limited bandwidth around a wavelength .lambda. approximately equal to 1550 nm. This is determined by the physical properties of the Erbium atoms.
In contrast, the gain of a SOA is determined by the semiconductor material. The central wavelength of the gain band is determined by the composition and structure of the semiconductor material. Thus, a SOA can be manufactured to provide gain at wavelengths not served by EDFAs. In addition, a SOA is not optically pumped. Gain is provided by applying a current to the device. Thus, a SOA is simpler, and more compact than an EDFA.
A variation of the typical SOA is a gain-clamped SOA (GC-SOA). It is advantageous to clamp the gain of a SOA because, when the gain is not clamped, it varies as a function of the total input signal power. When the input signal is made of multiple wavelength channels, this leads to cross-talk between wavelength channels.
The gain of a SOA increases with the number of carriers in the active section which are in an excited state. In a GC-SOA the carrier density is clamped by the lasing action at another wavelength. The gain at the lasing wavelength will equal the loss of the lasing cavity. This gain is approximately the same gain seen by the incoming signals. There is some wavelength dependence of gain, and the signals are at other wavelengths. The cavity loss is equal to the internal loss, due to effects such as absorption, plus the mirror loss. The mirror loss is equal to ln (1/R.sub.1 *R.sub.2)/2L, where ln denotes the natural logarithm, R.sub.1 and R.sub.2 denote the reflectivity of the mirrors at the lasing wavelength and L denotes the effective length of the cavity. As the current to the Bragg reflectors is varied, the reflectivity v. wavelength curve shifts.
Adjusting the gain by changing the current to the active section relies on the fact that the carrier density is imperfectly clamped. Non-radiative effects, such as Auger recombination, cause the carrier density to increase at higher current levels. These are higher-order effects, and for best device performance are usually minimized.
Wolfson et al. discloses an active GC-SOA (referred to as GC-SOA-A) where the Bragg reflectors are tuned in unison in "Detailed Theoretical Investigation of the Input Power Dynamic Range for Gain-Clamped Semiconductor Optical Amplifier Gates at 10 Gb/s", IEEE Photonics Technology Letters, Vol. 10, No 9, September 1998. This technique shifts the peak wavelength of the Bragg reflectors and the lasing wavelength. By shifting the lasing wavelength, the carrier density of the cavity must shift to preserve the lasing condition. The tuning currents also introduce a small amount of additional loss in the cavity. Therefore, tuning the lasing by tuning the reflective mirrors together results in a small change in the gain of the GC-SOA-A. A drawback to this gain control mechanism is that the lasing wavelength must vary by a significant amount to change the gain at the signal wavelengths. Therefore, the system may need to reserve a wide wavelength band for the lasing wavelength, and that band will not be available for use by the signal wavelengths.
Conventionally, the gain of GC-SOAs was typically adjusted by adjusting the current applied to the active section, or by adjusting the current to both mirror sections, so that they remained aligned. However, these methods alter the gain due to second or third order effects. Therefore, adjustment in this way is somewhat limited.
The effective gain of a GC-SOA can be adjusted using a variable attenuator in combination with the GC-SOA and adjusting the attenuation provided by the attenuator. By increasing the attenuation, output signal power is decreased. However, using a variable attenuator in conjunction with a GC-SOA is often detrimental to optical signal quality because the attenuator simply reduces the output power level. Moreover, adjusting the gain of a GC-SOA by adjusting the current applied to the active section of a GC-SOA affects the gain only through second or third order effects. Therefore, the user can only adjust the clamped gain by small amounts.